avoid typing enums as 'is Int'

This was perceived to be confusing, even if it's true.

Author: Carl Masak

type-graph.txt

@@ -170,14 +170,12 @@ class CurrentThreadScheduler    does Scheduler
 class ThreadPoolScheduler       does Scheduler
 class Promise
 class Promise::Vow
-# enum PromiseStatus
-enum PromiseStatus             is Int
+enum PromiseStatus
 class Channel
 class Tap
 role Supply
 class SupplyOperations
-# enum Signal
-class Signal                    is Int
+enum Signal
 class Semaphore
 class Lock
 class Proc::Async